You are wrong about the weight. The IS300 is much heavier than the IS200, approx. 150 kg.
Mostly because of its engine.
In addition, the IS300 is tuned for comfort rather than sporty/racy. Toyota Japan is promoting the Altezza or the Altezza with turbo charger (third-party option) for sporty/racy.
But yes, the stock IS300 has much more power than the stock IS200, so the stock IS300 is much faster.
However, compared to a Compressor-charged IS200 or a Turbocharged Altezza, the IS300 will likely lose the acceleration race. But, not with a lot, and the IS300 driver can do this without any sweat, while the others need to do some smart shifting.
The best thing of getting an IS300 is that everything is covered by the Factory :)